This invention relates to a torque sensor, in general, and to a magnetoelastic torque sensor and a method for making such a torque sensor, in particular.
There are many applications where it may be desirable to sense the torsional stress applied to a torque-carrying member without contacting the member. In one type of apparatus for doing this, the torque-carrying member is surrounded by a magnetoelastic material, and a magnetic field detector is disposed adjacent to the magnetoelastic material for sensing changes in a magnetic field that passes through the material. These changes are indicative of torsional stresses within the torque-carrying member.
This type of magnetoelastic torque sensor is difficult to manufacture, costly, fragile and/or not well suited for rough-duty uses such as in the automotive and industrial fields.
It is an object of the invention to provide an improved magnetoelastic torque sensor for measuring the magnitude of torque applied to a torque-carrying member and a method for making such a torque sensor.
According to a feature of the invention, a magnetoelastic torque sensor is provided with an inner flux guide with a projecting sense coil core.
According to another feature of the invention, a magnetoelastic torque sensor is provided with an inner flux guide made of magnetically conducting foil in a flat annular configuration with a projecting sense coil core tab.
It is yet another feature of the invention to simply form the inner flux guide, by cutting, stamping or etching to reduce production costs and provide other benefits.
It is further feature of the invention to provide a magnetoelastic torque sensor which utilizes a common material for both the inner flux guide and the sense coil core.
According to features of the invention, a non-contact, non-compliant torque sensor which is mechanically robust, highly reliable and extremely accurate is provided. A magnetoelastic element is disposed around the torque-carrying member and is fabricated by a thermal spraying process, wherein the magnetoelastic material is bonded onto the underlying torque-carrying member such as e.g. a torsion shaft member. In operation, torque applied to the shaft member is sensed by measuring changes in the magnetic field of the magnetoelastic material. These magnetic field changes occur in response to the torque applied to the shaft member which deforms the magnetoelastic material thereon resulting in a change of the magnetic field. The invention provides a torque sensor with a simple configuration and, with electronics of the magnetic pickup device, provides the sensor with unprecedented performance even when compared to more costly torque sensing devices.
The magnetoelastic torque sensor of the invention comprises an inner ferromagnetic flux guide encircling the shaft member in the vicinity of the magnetoelastic element, an outer ferromagnetic flux guide magnetically coupled to an outer edge of the magnetoelastic element, and a sense coil core, or preferably a plurality of sense coil cores, connecting the inner and outer flux guides. The sense coil core in conjunction with the flux guides acts as a main part of a magnetometer for measuring the magnetic flux from the flux generating source, namely the magnetoelastic material. The magnetic flux is collected and ducted to the sense coil cores via the inner and outer flux guides. Ferromagnetic material that has a square magnetic hysteresis loop is used for the cores of the sense coils. Amorphous metal materials (also commonly referred to as xe2x80x9cMetglasxe2x80x9d or xe2x80x9cglass transition metalxe2x80x9d) are preferably used at least for the construction of the sense coil cores.
According to another feature of the invention, in one embodiment an inner flux guide comprises a cylindrical ring structure (FIG. 5) fabricated from high permeability, low coercivity material generally referred to as Mu Metal. The ring structure has a pair of holes formed therein 180 degrees apart for receiving therein an amorphous wire which serves as the sense coil core. For the termination of the coil core to the outer flux guide which forms the flux return path, small notches are provided in the outer flux guide into which the amorphous wires extend.
According to a further feature of the invention, in a preferred embodiment a one-piece integral, inner flux guide and sense coil core is formed with at least one and preferably a single layer of amorphous metal foil having an annular configuration and at least one coil core tab in the same plane as that of the annular configuration.
The one-piece inner flux guide is supported, according to features of the invention, by cover and base pieces which are mounted about the shaft member so that the inner annular edge of the inner flux guide is slightly spaced apart from the magnetoelastic element. These cover and base pieces provide support for the metal foil, a substrate for printed electronic circuitry, mandrels of bobbins for centering the foil coil core tabs and for winding of the sense coils, and termination sites for coil wire connection to the electronic circuit while reducing the number of components to three. When assembled, the coil core tab is sandwiched between two complementary semi-cylindrical mandrel portions extending from the cover and base pieces. Coil wires are wound around the mandrel portions, which sense a change in the magnetic field of the magnetoelastic element as torque is applied to the shaft member.
Due to the one-piece amorphous metal foil serving as a combination flux guide and coil core material, a packaging is enabled with the single planar form of the foil. The flux guide foil formation is placed on a molded plastic carrier as the base piece, with its protruding tabs extending through the centers of the integral bobbins. A molded printed circuit board as the cover piece is positioned over the flux guide foil, with alignment features guaranteeing center hole concentricity and forming a sandwich to support and contain the amorphous foil formation. Molded protrusions from the circuit board complete the round winding mandrels of the bobbins, positioning the amorphous foil tabs in the center to serve as coil cores. The printed circuit board is extended in one region to allow the mounting and circuitry for interface electronic components and the electrical connector. This stack-together assembly greatly simplifies the manufacturing process and provides all of the essential features, the inner flux guide and its support (base and cover pieces), with essentially three elements.
According to a feature of the preferred embodiment of the invention, an approximately 0.001xe2x80x3 thick amorphous metal foil is laser cut or etched to form a flat ring with symmetrical, protruding coil core tabs spaced 180 degrees apart as the one-piece inner flux guide and sense coil core. This form of foil constitutes the inner flux guide for ducting flux from the source to be measured (e.g. the shaft and its magnetoelastic element which it encircles). It has been found that with very small cross sectional areas, even materials with relatively high magnetic saturation density characteristics will saturate at a modestly low point; also the saturation point will vary directly with cross sectional area. Given a constant material thickness (in this case about 0.001xe2x80x3), the saturation point will vary as a function of material width. An approximately 0.007xe2x80x3 diameter amorphous wire used as the sense coil core in the first embodiment is replaced by a strip of about 0.001xe2x80x3 thickness amorphous foil in the preferred embodiment. It has been found that a foil strip with a width of 0.040xe2x80x3 has a cross sectional area approximately equal to that of a 0.007 diameter wire, and produced similar performance to the wire core of the first embodiment when used as a coil core. The width of the foil tabs may range from about 0.030xe2x80x3 to 0.60xe2x80x3, and is optimum at 0.040xe2x80x3 (1 mm). The same parameters can be used with an amorphous material deposited on a suitable substrate by means of plating, thermal spraying, vapor deposition, and the like.
The one-piece amorphous foil formation of the inner flux guide of the present invention effectively improves the fabricated Mu Metal flux guide ring and the amorphous xe2x80x9cMetglasxe2x80x9d wire used in the first embodiment. While reducing the number of parts and the total cost, the preferred embodiment of the present invention allows greater freedom in packaging design and greatly simplifies the assembly techniques. The one-piece inner flux guide of the preferred embodiment is simple in construction and minimizes the manufacturing costs of producing a magnetometer of a torque sensor. In addition, the use of the amorphous foil as a coil core material simplifies the termination of the coil cores to the outer flux guide, and the need for forming wire notches in the outer flux guide of the first embodiment is eliminated. The foil tabs of the one-piece inner flux guide are thin and flexible enough to bend 90 degrees and so contact side walls of a pair of drawn cup-shaped pieces of the outer flux guide.
To serve as a flux guide and effectively conduct flux from the magnetoelastic material on the shaft to the sense coil cores, the preferred foil formation possesses a greater flux capacity in its annular section to ensure that the annular section does not saturate before the coil cores. Because a common material is used for both the inner flux guide and the coil cores, it was found important to use a strip of greater width in the annular section of the inner flux guide. It was found that a width of about 0.100xe2x80x3 was adequate for performance goals.
The one-piece inner flux guide is capable of gathering magnetic flux generated from the magnetoelastic material encircling the shaft and conducting the collected magnetic flux to its one-piece coil core tabs. Although multiple layers of amorphous foil can be utilized, to keep fabrication to its simplest form, a single layer of foil is preferably employed since it performs as well as multiple layers.
The first and preferred embodiments of the present torque sensor function in the same fashion. Both embodiments differing essentially in the configuration of their inner flux guides.
The flux guide assembly of the invention is further provided with outer flux guides constructed of Mu Metal to form the flux return path. The flux guide assembly acts to provide a flux density gain by concentrating the magnetic signal into the area of the pickup devices, to integrate irregularities out of the signal being measured by collecting the magnetic flux over a larger angular distance, to shield the magnetic signal from the influence of stray magnetic fields, and to shield the pickup devices from electromagnetic interference. The geometric placement of the magnetic pickup devices (coils located between the flux concentrators on opposite sides of the magnetoelastic element) creates a common mode rejection configuration which cancels the effects of stray fields which pass through both pickup devices in the same direction.
The electronic, circular magnetometer of the invention measures the strength of the magnetic field emanating outward (or inward) from the shaft, at the circumferential centerline of the magnetoelastic element. The torque sensor of the present invention has various uses including, but not limited to, automotive technologies. The automotive applications include steering wheel applications in electric power steering systems, as well as crankshaft torque measurements, anti-lock brake system wheel torque measurements, vehicle suspension measurements, and brake pedal torque measurements for electric braking.
According to a further feature of the invention, the magnetoelastic element made of a high nickel content powered metal is thermally sprayed onto the shaft member. This thermal spray process fuses the coating to the shaft member. The thermal spray coating provides a typical bond strength in the order of 10,000 psi or greater. In this manner, the present invention improves the ability of the magnetoelastic element to form an intimate bond with the shaft member capable of surviving high torque levels. The integrity of this interface is crucial to the stability of the magnetoelastic properties. In addition, the thermal spray process achieves a high level of uniformity in the sprayed material density, chemical composition, internal stresses, and the surface finish and translates into exceptional rotational regularity of the magnetic signal. The homogeneity of the thermally sprayed metal properties directly relates to consistency in the magnetic performance of the magnetoelastic element. The composition of the sprayed material in conjunction with spray parameters (particle size, particle velocity, powder feed rate, etc.) may be selected so as to foster maximum hardness, low levels of oxides, and low porosity, and to yield a magnetoelastic element with optimum resistance to the effects of corrosives and to stress cracking.